1. Field
The invention pertains to a process for imparting a lightfast, corrosion resistant coating to aluminum or aluminum alloy.
2. State of the Art
Many attempts have been made in the past to produce a lightfast green coating on anodized aluminum or aluminum alloys by use of organic dyes, inorganic pigments, and other means. Each of the prior art methods has had its drawbacks. Organic coatings which have been used eventually chalk or blister. Green organic dyes which have been used for at least thirty (30) years to dye anodic films, all have very poor lightfastness and none are suitable for long term outdoor weathering for such things as signs and buildings. The green dyes are only suitable for novelty items and products which need not be colorfast.
Attempts have also been made to use electrolytic means to produce colors on aluminum and aluminum alloys. U.S. Pat. No. 3,616,309 discloses a process using an electrolyte containing sodium tellurite. The color produced, however, is weak and of a pale light yellow-green shade having no commercial value for signs or architectural purposes. A closely related process using an electrolyte containing tellurium is disclosed in U.S. Pat. No. 3,616,308, and the results are about the same as those achieved using the process of the previously cited patent. Another process for coloring anodic coatings on aluminum and aluminum alloys is disclosed in U.S. Pat. No. 3,661,729 wherein the aluminum or an alloy thereof is first subjected to anodic oxidation using an electric current comprising both a direct and alternating current component. The resulting oxide coating is then dipped in an aqueous solution of metallic cations or anions in the absence of any electric current. A very poor yellow-green and a light pea green color is produced erratically by this process. The colors so obtained are not suitable for road signs or other architectural purposes.